1. Field of the Invention
The present invention relates to modifying material. Preferred embodiments of the invention relate to perceptible modification.
“Material” as used herein means information material which includes at least one or more of image material, audio material and data material. Image material is generic to still and moving images and includes video and other forms of information signals representing images.
2. Description of the Prior Art
It is known to watermark material. A watermark may be data embedded in material as a watermark. A watermark may be perceptible or hidden. Preferred embodiments of the present invention use perceptible watermarks.
It is also known to watermark an image by transforming the image from the spatial domain to a transform domain, e.g. to the wavelet domain and embed a watermark by changing the wavelet coefficients. The transform domain image with the watermark is then inverse transformed to the spatial domain. See for example “An image watermarking method based on the wavelet transform” by Hisashi Inoue, et al, IEEE, 0-7803-5467-2/99.
U.S. Pat. No. 5,809,139 (Girod et al) discloses the invisible watermarking of video material which is already compressed. Video which has been compressed by MPEG2 is entropy decoded and dequantised to obtain the DCT coefficients. A spread spectrum spatial domain watermark is transformed to the DCT domain and the transformed watermark added to the DCT coefficients of the transformed video. To maintain the bit rate of the compressed video even with the addition of the watermark, various steps are taken. Only non-zero coefficients are modified. Also the number of bits of an encoded watermarked coefficient is compared against the number of bits of the coefficient prior to watermarking (plus any extra number of spare bits saved in previous coding operations). A control controls selection of the output between the watermarked bitstream and the original unwatermarked bitstream. If an entropy encoded unwatermarked coefficient uses no bits and the watermarked entropy encoded coefficient uses n1 bits then a watermarked coefficient is output if n1<=n0+n3, where n3 is the number of spare bits available for use. The watermark can be detected in a video decoder but it is not possible to restore the watermarked video to the original video by reversing the watermarking algorithm.
WO 99/10837 (Digimarc) discloses a method of watermarking a video image in which the image is compressed using for example MPEG2 and the DCT coefficients are modified to embed a watermark. The bit rate is preserved by maintaining a count that represents the amount the bit rate has been increased by the modifications to the coefficients less the amount the bit rate has been decreased by the modifications. If the cumulative change exceeds a positive or negative limit, then no further changes are made. The watermark can be detected in a video decoder but it is not possible to restore the watermarked video to the original video by reversing the watermarking algorithm.
Both U.S. Pat. No. 5,809,139 and WO 99/10837 require means which monitor the bit rate and which stops embedding of a watermark if a specified limit is reached. For U.S. Pat. No. 5,809,139 and WO 99/10837 that results in a random distribution of the watermark in an image. These proposals are aimed at irreversible imperceptible watermarks which embed data representing a message in the image.
It is known to compression encode material especially images. Also, if a modification is made to the material which adds data to the material, then the compression is usually less efficient. It is desirable to process compressed material, so as to produce modification of the material which is perceptible in the decompressed material, e.g. a visible modification in an image, with a minimal change to the number of bits representing the compressed material. Preferably it is desirable to modify material in a way which is easily applicable to both material which is already compressed and material which is to be compressed.